Discovery provides evidence of new therapeutic target that could delay axon decay
Degeneration of the axon and synapse, the slender projection through which neurons transmit electrical impulses to neighboring cells, is a hallmark of some of the most crippling neurodegenerative and brain diseases such as amyotrophic lateral sclerosis (ALS), Huntington's disease and peripheral neuropathy. Scientists have worked for decades to understand axonal degeneration and its relation to these diseases. Now, researchers at the University of Massachusetts Medical School are the first to describe a gene - dSarm/Sarm1 - responsible for actively promoting axon destruction after injury. The research, published today online by Science, provides evidence of an exciting new therapeutic target that could be used to delay or even stop axon decay.
"This discovery has the potential to have a profound impact on our understanding of neurodegenerative diseases, much like the discovery of apoptosis (programmed cell death) fundamentally changed our understanding of cancer," said Marc R. Freeman, PhD, associate professor of neurobiology at the University of Massachusetts Medical School and lead investigator on the study. "Identification of this gene allows us to start asking exciting new questions about the role of axon death in neurodegenerative diseases. For example, is it possible that these pathways are being inappropriately activated to cause premature axon death?"
For more than a century, scientists believed that injured axons severed from the neuron cell body passively wasted away due to a lack of nutrients. However, a mouse mutation identified in the early 1990s - called slow Wallerian degeneration (Wlds) - was able to suppress axon degeneration for weeks. This finding forced scientists to reassess Wallerian degeneration, the process through which an injured axon degenerates, as a passive process and consider the possibility that an active program of axon auto-destruction, akin to apoptotic death, was at work instead.
If Wallerian degeneration was an active process, hypothesized Dr. Freeman, a Howard Hughes Medical Institute Early Career Scientist, then it should be possible through forward genetic screens in Drosophila to identify mutants exhibiting Wlds-like axon protection. Freeman and colleagues screened more than 2,000 Drosophila mutants for ones that exhibited long-term survival of severed axons. Freeman says this was a heroic effort on the part of his colleagues. The screen took place over the next two and a half years, and involved seven students and post-docs in the Freeman lab-Jeannette M. Osterloh, A. Nicole Fox, PhD, Michelle A. Avery, PhD, Rachel Hackett, Mary A. Logan, PhD, Jennifer M. MacDonald, Jennifer S. Zeigenfuss-who performed the painstaking and labor-intensive experiments needed on each Drosophila mutant to identify flies that suppressed axonal degeneration after nerve injury.